The results of three ambient air monitoring programs performed downwind of The Geysers, California, are described. These studies, conducted since 1976, have monitored the declining ambient air concentrations of hydrogen sulfide CH2S) m Lake County. During the 13 years of monitoring, geothermal power production has increased from approximately 500 to 2,000 MW, H2S emissions from power plants have declined from 1,900 to less than 200 lb/hr, and ambient H2S concentrations have significantly declined. Annual average concentrations of H2S at four long-term sites have declined by a factor of 3.0, maximum H2S concentrations have declined by a factor of 3.6, and the frequency of violation of the California Air Quality Standard (0.03 ppm) averaged over 1 hour has declined from an average frequency of 52 times per year to a1most 0. The area has not had a recorded violation of the air quality standard since August 1987. As such, the area has gone 3 years without a violation was classified by the California Air Resources Board as “attainment” during their November 1990 review process.